Update readme

Author: zauonlok

README.md

@@ -47,9 +47,13 @@ run `build_win32.bat`.
 
 ### macOS
 
-Install [Xcode](https://itunes.apple.com/us/app/xcode/id497799835?mt=12) and
+Install Command Line Tools for Xcode with the command below and
 run `build_macos.sh`.
 
+```
+xcode-select --install
+```
+
 ### Linux
 
 Install GCC and Xlib with the following commands and run `build_linux.sh`.